Low-voltage industrial electrical systems characterized by high currents and power levels normally use specific devices, commonly known in the art as automatic power circuit breakers.
These circuit breakers are designed so as to provide a series of features required to ensure the correct operation of the electrical system in which they are inserted and of the loads connected to it. For example, they ensure the nominal current required for the various users, allow correct insertion and disconnection of the loads with respect to the circuit, protect the loads against abnormal events such as overloading and short-circuits by opening the circuit automatically, and allow to disconnect the protected circuit by galvanic separation or by opening suitable contacts in order to achieve full isolation of the load with respect to the electric power source.
Currently, these circuit breakers are available according to various industrial embodiments, the most common of which entrusts the opening of the contacts to complicated kinematic mechanisms that utilize the mechanical energy stored beforehand in special opening springs and are generally triggered, in case of electrical fault, by an appropriate protection device, typically a relay.
In certain operating conditions, particularly when the presumed short-circuit current can assume significantly high values, the use of devices that utilize in a traditional manner the energy that can be accumulated in the opening springs can be scarcely efficient and uneconomical for opening the contacts; in such cases, one normally resorts to special types of automatic circuit breaker that have technical solutions aimed at increasing their breaking capacity.
Among the technical solutions that are currently most widely used, there are two that are often used in combination. In particular, a first solution forces the current to follow a given path, so that when a short circuit occurs, electrodynamic repulsion forces occur between the contacts. These repulsion forces generate a useful thrust that helps to increase the separation speed of the moving contacts with respect to the fixed contacts; in this manner, the intervention time is reduced and the presumed short-circuit current is prevented from reaching its maximum value.
The second solution doubles the fixed contacts and the moving contacts. In this case, the flow of current is interrupted in each pole of the circuit breaker in two separate regions that are arranged electrically in series to each other, so that each region is subjected to a lower mechanical and thermal stress.
A particularly critical aspect of known types of circuit breaker is the fact that the presence of electrodynamic repulsion forces, while contributing positively to the generation of the thrust useful for contact separation on the one hand, on the other hand helps the moving contact structure to reach the end of its stroke at high speed and therefore with great energy. This generally tends to cause violent impacts against the case of the circuit breaker, with the possibility of damaging it, and can therefore require the use of additional cushioning elements; moreover, bouncing of the moving contacts toward the fixed contacts and undesirable restrikes of the electric arc can occur. In the case of circuit breakers with double contacts, the likelihood of bouncing and restriking of the electric arc can be increased by the presence of additional springs, which are usually associated with the structure of each moving contact in order to facilitate an even distribution of the mechanical pressure on the two surfaces for coupling between each moving contact and the corresponding fixed contacts.